[RFCv2 0/9] UEFI emulator for kexec

[Pingfan Liu]

On Thu, Aug 29, 2024 at 1:08 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>
[...]
>
> Thanks for putting this RFC together. This is useful work, and gives
> us food for thought and discussion.
>
> There are a few problems that become apparent when going through these changes.
>
> 1. Implementing UEFI entirely is intractable, and unnecessary.
> Implementing the subset of UEFI that is actually needed to boot Linux
> *is* tractable, though, but we need to work together to write this
> down somewhere.
>   - the EFI stub needs the boot services for the EFI memory map and
> the allocation routines
>   - GRUB needs block I/O
>   - systemd-stub/UKI needs file I/O to look for sidecars
>   - etc etc
>

I have created a git repo to hold the record for the current status.
[https://github.com/rhkdump/kexec_uefi.git]
And uefi_subset.md records the minimal requirement of uefi.

But I have a question about "GRUB needs block I/O", is it required? As
I know, the kernel image e.g. UKI, zboot will be supported. But why
should grub be supported too?

Thanks,

Pingfan

> I implemented a Rust 'efiloader' crate a while ago that encapsulates
> most of this (it can boot Linux/arm64 on QEMU and boot x86 via GRUB in
> user space **). Adding file I/O to this should be straight-forward -
> as Lennart points out, we only need the protocol, it doesn't need to
> be backed by an actual file system, it just needs to be able to expose
> other files in the right way.
>
> 2. Running the UEFI emulator on bare metal is not going to scale.
> Cloning UART driver code and MMU code etc is a can of worms that you
> want to leave closed. And as Lennart points out, there is other
> hardware (TPM) that needs to be accessible as well. Providing a
> separate set of drivers for all hardware that the EFI emulator may
> need to access is not a tractable problem either.
>
> The fix for this, as I see it, is to run the EFI emulator in user
> space, to the point where the payload calls ExitBootServices(). This
> will allow all I/O and memory protocol to be implemented trivially,
> using C library routines. I have a crude prototype** of this running
> to the point where ExitBootServices() is called (and then it crashes).
> The tricky yet interesting bit here is how we migrate a chunk of user
> space memory to the bare metal context that will be created by the
> kexec syscall later (in which the call to ExitBootServices() would
> return and proceed with the boot). But the principle is rather
> straight-forward, and would permit us, e.g., to kexec an OS installer
> too.
>
> 3. We need to figure out how to support TPM and PCRs in the context of
> kexec. This is a fundamental issue with verified boot, given that the
> kexec PCR state is necessarily different from the boot state, and so
> we cannot reuse the TPM directly if we want to pretend that we are
> doing an ordinary boot in kexec. The alternative is to leave the TPM
> in a state where the kexec kernel can access its sealed secrets, and
> mock up the TCG2 EFI protocols using a shim that sits between the TPM
> hardware (as the real TCG2 protocols will be long gone) and the EFI
> payload. But as I said, this is a fundamental issue, as the ability to
> pretend that a kexec boot is a pristine boot would mean that verified
> boot is broken.
>
>
> As future work, I'd like to propose to collaborate on some alignment
> regarding a UEFI baseline for Linux, i.e., the parts that we actually
> need to boot Linux.
>
> For this series in particular, I don't see a way forward where we
> adopt this approach, and carry all this code inside the kernel.
>
> Thanks.
> Ard.
>